Electric blasting fuse



Dec. 15, 1931. J. SCHURMANN ELECTRIC BLASTING FUSE Filed Nov, 19, 1928 2 Sheets-Sheet 1 W B\D my Inventor.-

Dec. 15, 1931. J. SCHURMANN ELECTRIC BLASTING FUSE Filed Nov. 19, 1928 2 Sheets-Sheet 2 Fig.7:-

Patented Dec. 15, 1931 UNITED STATES PATENT OFFICE JULIUS SCH'U'BHANN, OF BEBLIN-ZEHLENDORF, GERMANY, ASSIGNOB TO THE FIRM FABBIK ELEKTBIBGHER ZUNDEB G. M. B. 'H., COLOGNE-m, GERMANY ELECTRIC BLASTING FUSE Application filed November 19, 1928, Serial No. 320,448, and in Germany July 18, 1928.

Electric blasting fuses have recently been required to' have a blasting voltage in excess of a prescribed minimum value in order to ensure sufiicient security against stray currents and possible other undesired electrical currents, that is to say that the fuse shall only ignite, if the difierence of potential at the fuse wires exceeds or volts or other prescribed voltage and at the same time re- 10 quire a current of 100, 150, 200 and more milli-amperes. To attain this end it has been proposed to increase the resistance of the i 1tion wire embedded v in the priming c arge (which may be either a match-head 16 or a loose mass), for example, by making the ignition wire exceptionally long, andin this manner the difference of potential at the ends of the ignition wires required for blastin has been raised to a certain degree.

99 en such fuses are subjected to a lower voltage than the blasting volta e, of course, a smaller current than theblastmg current is obtained, but as the heat generated by this current accumulates in the priming charge,

the ignition temperature is finally obtained even with the smaller currents and the fuse is ignited if the small current is maintained long enough. As a stray current, contrary to the blasting current, may act for an unlimited period, such fuses are therefore only pseudoproof against stray currents.

According to the present invention, se-

curity against constant stray currents is attained with an ignition wire which may be of normal resistance, for example, of normal length and diameter, by interposing a resistance outside the riming charge and between the current lea s and theignition wire so that the heat of the current generated in this interposed resistance cannot reach the priming charge. The interposed resistance is preferably disposed as near to the ignition wire as compatible with these conditions, for example, it is arranged in the fuse cartridge itself or in an outer case or wound upon t e cartridge or outer case or upon both. This is done to shorten as much as possible that part of the current lead through which any stray current might enter the ignition wire without passing through the interposed resistance.

The accompanying illustrations show examples of the construction of fuses accord- 111%!30 the invention to an enlarged'scale.

1g. 1 shows a match-head fuse, partly in section, with a resistance wound upon the pole strip.

Fig. 2 shows a match-head fuse in section with an interposed resistance inserted in the pole strip.

Figs. 3 and 4 show match-head fuses in section with resistance fitted upon the pole strip.

Fig. 5 is a section through a blasting cartridge with an interposed resistance inserted in one of the current leads.

Fig. 6 is a detail sectional view, and Fig. 7 a sidev view of parts shown in Fig. 5.

In the example shown in Fi 1, the numeral 1 designates the usual p0 e strip, i. e. a strip of insulating material coveredon both sides with the usual conducting foils 2 and 2. The numeral 3 designates the ignition wire, 4 the priming charge and 5 and 5' the usual current leads soldered to the strips 2 and 2'. The strip 2 connects the lead 5 to the ignition wire 3 in the usual manner, but the connection between the lead 5' and the ignition wire3 is interrupted.

Below the composition 4, insulation 6 is placed around the covered strip 1 and a resistance wire 7 is wound upon the insulation 6. One end of the wire 7 is attached at 7 to the lower part of the covering strip 2' and connected through it to the lead 5' whilst the other end of the wire 7 is attached at 7" to the upper part of the covering strip 2' and through it with the ignition wire 3, so that the wire 7 is interposed between the lead 5' and the ignition wire 3.

In the example shown in Fig. 2, the connection made between the ignition wire and one of the leads through the stri 2' is not only broken to a certaln extent, ut the insulating strip 1 is also provided at the point of interruption with a recess. If the strip 1 is composed in approved manner of two strips 1 and 1" each coated on the outside with a conducting covering, the recess may be formed e. g. by slotting the strip 1". An insulating strip 8 is inserted in the slot and the resistance wire 7 is wound on the strip 8 and connects the lead to the ignition wire 3.

In the example shown in Fig. 3, the resistance wire is carried by a bobbm 9 of insulatin material fitted upon the stri 1. The

b0 bin 9 may also be covered wit an additional insulating coating e. g. of high in sulating or heat resisting Iproperties.

The example shown in i g. 4 is similar in construction to that shownm Fig. 8 but the covered strip 1 is longer by the distance a between the u per end of the resistance wire 7 and the un erside of the composition 4 in order to retard the transmission of heat from the resistance wire 7 to the'composition 4.

In the construction according to Fig. 5,

the fuse head is constructed in the usual mannet and fixed in the blasting cartridge 11. Outside the cartridge 11 the current lead 5' is broken and a resistance 12 is insertedin the break. This resistance consists of an insulating .511 port 13, made for example as shown in Big. 6 from a cardboard strip on both sides of which is laid a brass stri 14. Around the strip 13 and outside the rass strips 14 is laid an insulating covering 15 upon which 'a resistance wire 16 is wound and the whole is enclosed'in insulating material 17. The brass strips 14 are doubled over at opposite ends to grip the ends 15' of the resistance wire, which for greater security are also soldered to the strips 14. The free ends 14' of the brass strips are bent out and referabl drilled to receive the broken en s of the cad-5 and also to enable them to be more surely soldered thereto.

The resistance 12 and the lead 5 are strengthened and more perfectly insulated by enclosing them in insulating and refractory material, e. g. by winding asbestos strip or cord 18 around them jointly or by enclosingthem in an insulating sleeve of asbestos or other insulating material.

1.1 the form shown in Fig. 5 the blasting fuse 11 is inserted in an outer case 19 which is so long that it extends beyond the resistance 12 to such a distance that the current leads 5 and 5' twisted together below the re- 50 sistance 12 can be sealed in the outer case 19 by means of a plug 20 and sealing compound It will be apparent that the fuse is mechanically protected at its most delicate portion,

i. e. between the match-head and the resistance, against any touch with other current wires, and any stray current entering into the lead 5 or 5' at any other pointperhaps by fault of insulatione-cannot reach 0 the ignition wire without passing through the interposed resistance.

The insulating material and as far as possible all the sealing compounds employed in the fuse are referably of non-inflammable material so at the fuse is not only proof ployed, the

-match-head type, since plicable to electric blasting fuses of an 0011- c arge.

struction or'arrangement of primi Having now particularly descri and ascertained the nature of my said. invention and in what manner the same is to be performed I declare that'what' I claim is 1. An electrically ignited blasting fuse comprising a fuse casi containing a priming charge, an electrica ignition member in the charge and conducting elements to said member; in combination with a tube ti htly surrounding the casing, current leads thereon, and a resistance element in said tube included between one of -said leads and the conductin elements.

2. An e ectrically ignited blasting fuse, comprising a fuse casing, a priming charge and its support, an electric ignition member in the charge and current leads to said member all cemented in said casin in combination with a tube contacting with said casing and a resistance element in said tube and in one of said leads, and means for sealing said leads in said tube to inclose the resistance element between the casing and sealing means.

3. An electrically ignited blasting fuse comprising a support, electrically conducting leads-on opposite sides of said support, a prlming charge connecting said leads, an igniting element in the charge bridging the leads, an insulating support close to said charge and a resistance windin on the support and included in oneof t e leads, all closely arranged and forming a unit for insertion in a blasting charge.

In testimony whereof I aflix my signature.

JULIUS SGHURMANN. 

